1. Field of the Invention
The present invention relates to a clamping mechanism that secures a workpiece to a mechanical arm. More particularly, the present invention relates to a clamp that gently secures a semiconductor wafer to a robot blade by biasing the wafer against a retaining member at the forward edge of the blade when the robot blade is at least partially retracted for rotation. The clamp is actuated by a pneumatic cylinder and utilizes a flexure member to maintain a desirable clamping force against the wafer.
2. Background of the Related Art
Modern semiconductor processing systems include cluster tools which integrate a number of process chambers together in order to perform several sequential processing steps without removing the substrate from a highly controlled processing environment. These chambers may include, for example, degas chambers, substrate preconditioning chambers, cooldown chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, etch chambers, and the like. The combination of chambers in a cluster tool, as well as the operating conditions and parameters under which these chambers are run, are selected to fabricate specific structures using a specific process recipe and process flow.
Once the cluster tool has been set up with a desired set of chambers and auxiliary equipment for performing certain process steps, the cluster tool will typically process a large number of substrates by continuously passing substrates through a series of chambers and process steps. The process recipes and sequences will typically be programmed into a microprocessor controller that will direct, control, and monitor the processing of each substrate through the cluster tool. Once an entire cassette of wafers has been successfully processed through the cluster tool, the cassette may be passed to yet another cluster tool or stand alone tool, such as a chemical mechanical polisher, for further processing.
Typical cluster tools process substrates by passing the substrates through a series of process chambers. In these systems, a robot is used to pass the wafers through a series of processing chambers. Each of the processing chambers is constructed to accommodate and process two wafers at a time. In this way, throughput of substrates in the cluster tool is effectively doubled. The amount of time required by each process and handling step has a direct impact on the throughput of substrates per unit of time. While the exact design of an integrated circuit fabrication system may be complex, it is almost always beneficial to perform each step as quickly as possible to maximize overall throughput without detrimentally affecting product quality, operating costs, or the life of the equipment.
Substrate throughput in a cluster tool can be improved by increasing the speed of the wafer handling robot positioned in the transfer chamber. As shown in FIG. 1, the magnetically coupled robot comprises a frog-leg type connection or arms between the magnetic clamps and the wafer blades to provide both radial and rotational movement of the robot blades in a fixed plane. Radial and rotational movements can be coordinated or combined in order to pick up, transfer, and deliver substrates from one location within the cluster tool to another, such as from one chamber to an adjacent chamber.
Another exemplary robot is shown in FIG. 2. FIG. 2 shows a conventional polar robot with an embodiment of the substrate clamping apparatus of the present invention. As shown in FIG. 2, like the xe2x80x9cfrog-legxe2x80x9d type robot of FIG. 1, radial and rotational movements may be coordinated or combined in order to pick up, transfer, and deliver substrates from one location within a cluster tool to another, such as from one chamber to an adjacent chamber. However, unlike the robot in FIG. 1, the robot shown in FIG. 2 may also provide translational movement of wafer 302.
As the robot speed and acceleration increase, the amount of time spent handling each substrate and delivering each substrate to its next destination is decreased. However, the desire for speed must be balanced against the possibility of damaging the substrate or the films formed thereon. If a robot moves a substrate too abruptly, or rotates the wafer blade too fast, then the wafer may slide off the blade, potentially damaging both the wafer and the chamber or robot. Further, sliding movements of the substrate on the wafer blade may create particle contaminants which, if received on a substrate, can contaminate one or more die and, thereby, reduce the die yield from a substrate. In addition, movement of the substrate on the wafer blade may cause substantial misalignment of the substrate that may result in inaccurate processing or even additional particle generation when the substrate is later aligned on the support member in the chamber.
The robot blade is typically made with a wafer bridge on the distal end of the wafer blade that extends upwardly to restrain the wafer from slipping over the end. However, the wafer bridge does not extend around the sides of the blade and does very little to prevent the wafer from slipping laterally on the blade. Furthermore, the wafers are not always perfectly positioned against the bridge. Sudden movement or high rotational speeds may throw the wafer against the bridge and cause damage to the wafer or cause the wafer to slip over the bridge and/or off the blade.
There is a certain amount of friction that exists between the bottom surface of a wafer and the top surface of the wafer blade that resists slippage of the wafer. However, the bottom surface of a silicon wafer is very smooth and has a low coefficient of friction with the wafer blade, which is typically made of nickel plated aluminum, stainless steel or ceramic. Furthermore, a typical wafer is so lightweight that the total resistance due to friction is easily exceeded by the centrifugal forces applied during rapid rotation of the robot, even when the blade is in the fully retracted position. However, this low coefficient of friction is typically relied upon when determining the speed at which a robot rotates.
Patent application Ser. No. 08/935,293, entitled xe2x80x9cSubstrate Clamping Apparatus,xe2x80x9d filed on Sep. 22, 1997, which is hereby incorporated by reference discusses the problem of wafer slippage on a robot blade and the need to increase wafer transfer speeds. This application describes a clamping mechanism that holds the substrate on the blade during transfer. However, that invention is directed to a complex lever/flexure system to engage and disengage the clamp fingers.
Prior substrate clamping apparatus have also included pneumatically actuated clamp fingers in which a clamp finger assembly is actuated electronically through use of a solenoid when it is programmatically determined based on robot arm sensors that the robot arm is in the extended position. Such prior apparatus do not utilize flexure members in the gripping mechanism and may, accordingly, exert undue clamping forces against the wafer being secured to the blade. Such undue clamping forces may require moving parts such as bearings or slides to minimize particle generation upon engagement with the wafer. Such prior apparatus may utilize extension springs, compression springs, or other biasing members besides flexure members, which may generate more undesirable particles than use of flexure members.
There is a need for a robot that can transfer wafers at increased speeds and acceleration/decelerations, particularly in a multiple or single substrate processing system. More specifically, there is a need for a wafer clamping mechanism on a robot that can secure a wafer or a pair of wafers on a wafer blade or a pair of wafer blades with sufficient force to prevent wafer slippage and wafer damage during rapid rotation and radial movement while minimizing or eliminating undesirable particle generation.
In one aspect, the invention is directed to a clamp wrist for a robot assembly having one or more arms and one or more actuators for driving the arms to handle a workpiece, comprising: a wrist housing pivotally coupled to the arms; at least one clamp finger disposed in the wrist housing; and a biasing member coupled to the at least one clamp finger for urging the at least one clamp finger against the workpiece. A particular feature of this aspect of the invention is that the actuator may be a pneumatic cylinder. Further, the clamp finger may comprise a yoke, operatively connected to a piston rod of the pneumatic cylinder, and the yoke may be further operatively connected to at least one flexure member. Further, the flexure member may be connected to a tip end for engagement with an edge of the workpiece.
In another aspect, the invention may be directed to a clamping mechanism for securing a workpiece to a workpiece handling member coupled to the distal end of a robot arm, the workpiece handling member comprising a wafer handling blade having a workpiece receiving region and a retaining member at the distal end thereof, comprising at least one clamp finger adapted and positioned to contact the edge of the workpiece; and a biasing member coupled to the at least one clamp finger adapted to urge the at least one clamp finger against the workpiece when the workpiece is positioned on the workpiece receiving region to clamp the workpiece between the at least one clamp finger and the retaining member. A particular feature of this aspect of the invention is that the at least one clamp finger may further comprise a flexure assembly. The clamping mechanism may further comprise a pneumatic cylinder operatively connected to the flexure assembly to move the flexure assembly away from the wafer upon providing compressed air to the pneumatic cylinder. Still further, the flexure assembly may comprise: a yoke; a pair of tip ends; a flexure member connected between the pair of tip ends; and a tip flexure member connected between each of the tip ends and opposing apogee ends of the yoke. Another feature of the present invention is that the flexure member may also be connected proximate a medial point along the flexure member to the wrist housing, and the piston rod of the pneumatic cylinder may be rotatably mounted to the yoke so that the yoke is free to rotate about the axis of the piston rod.
In still another aspect, the invention may be directed to a robot arm assembly, comprising: a pair of frog-leg type robot arms, each arm having a distal end with a clamp wrist attached thereto; the clamp wrist comprising a wrist housing pivotally coupled to the robot arm; a flexure assembly disposed in the wrist housing adapted to positively grip a wafer; and a pneumatic cylinder disposed in the wrist housing and operatively connected to the flexure assembly to cause the flexure assembly to flex away from the wafer being gripped. A feature of this aspect of the invention is that the flexure assembly may be adapted to flex outwardly and rearwardly away from the wafer upon engagement of the flexure assembly by the pneumatic cylinder, and the flexure assembly may include at least one leaf spring. Another feature of this aspect of the invention is that the flexure assembly may be rotatably connected to a piston rod of the pneumatic cylinder. Still another feature of this aspect of the invention is that at least one of the flexure members may be affixed to the wrist housing to cause the tip ends to rotate outwardly as the flexure assembly is engaged by the pneumatic cylinder.